A condenser microphone is configured so that an electrostatic acousto-electric converter in which a diaphragm and a backplate are arranged opposedly is included in a microphone unit, and an impedance converter such as a field effect transistor (FET) is incorporated because of its very high impedance. Usually, in the condenser microphone, a phantom power source is used, and microphone sound signals are delivered via a balanced shielded cable for a phantom power source.
To connect the balanced shielded cable, a 3-pin type output connector specified in EIAJ RC-5236 “Latch Lock Type Round Connector for Audio Equipment” is provided on the microphone casing (microphone grip for handheld microphone) side. By this output connector, the balanced shielded cable can be attached to and detached from the condenser microphone.
If strong electromagnetic waves radiated from a cellular phone or the like are applied to a microphone or a microphone cable in the state in which the balanced shielded cable is connected to the output connector, the electromagnetic waves intrude into the microphone casing through the output connector, and sometimes are detected and delivered from the microphone as audible frequency noise.
As one method for preventing this phenomenon, the present applicant has proposed an output connector of microphone having a shielding function in Patent Document 1 (Japanese Patent Application Publication No. 2005-311752). The configuration of this output connector is explained with reference to FIGS. 3A to 3C. FIG. 3A is a front view of the output connector, in which only a shield cover is shown as a cross section, FIG. 3B is a sectional view taken along the line B-B of FIG. 3A, and FIG. 3C is a plan view of the output connector shown in FIG. 3A.
An output connector 10 includes a disc-shaped connector base 11 consisting of an electrical insulator such as PBT (polybutylene terephthalate) resin. In the connector base 11, three pins of a first pin E for earthing, a second pin SH on the hot side of signal, and a third pin SC on the cold side of signal are penetratingly provided, for example, by press fit.
In this specification, the first pin E for earthing is sometimes referred simply to as the “earthing pin E”, and the second pin SH on the hot side of signal and the third pin SC on the cold side of signal are sometimes referred simply to as the “signal pin SH” and “signal pin SC”, respectively.
As for the handheld condenser microphone, as shown in FIG. 3B, the output connector 10 is mounted in an end portion of a microphone casing (microphone grip for the handheld microphone), not shown, via a connector housing 20. The connector housing 20 consists of a cylindrical body made of a metal such as a brass alloy, and also functions as the shield casing of the output connector.
In the connector base 11, an internally threaded hole 13 is formed toward the radial direction from the outer peripheral surface thereof. In the internally threaded hole 13, a male screw 12 for fixing the output connector 10 to the connector housing 20 is threadedly mounted.
According this configuration, as shown in FIG. 3B, by using a screwdriver or the like, not shown, through a round hole 21 formed in the connector housing 20, the male screw 12 is turned, pulled out to the outside in the radial direction, and caused to butt against the peripheral edge of the round hole 21. Thereby, the output connector 10 can be fixed firmly to the connector housing 20.
For example, as shown in Japanese Patent Application Publication No. 2005-311752 to prevent electromagnetic waves radiated from a cellular phone or the like from intruding into the microphone casing, a circuit board 14 and a shield cover 15 are provided on the base inner surface (the surface arranged on the inside of microphone, the upper surface in FIGS. 3A and 3B) side of the connector base 11.
Although not shown in the figures in detail, the circuit board 14 is a double-sided printed wiring board having three through holes through which the earthing pin E and the signal pins SH and SC are penetrated. On the back surface side opposed to the base inner surface of the connector base 11, a shield layer consisting of a copper foil solid pattern is formed. On the upper surface on the opposite side thereof, a capacitor element for blocking intrusion of high-frequency signals and a Zener diode element for preventing circuit destruction due to static electricity are mounted. For convenience in drawing the figures, only the capacitor element is shown in FIG. 3B by reference symbol 14a. 
The capacitor element 14a and the Zener diode element are connected in parallel to between the earthing pin E and the signal pin SH, and further connected to between the earthing pin E and the signal pin SC. The capacitor element 14a functions as a bypass capacitor that allows a high-frequency current, which is caused by extraneous electromagnetic waves going to intrude into the microphone casing through the signal pins SH and SC, to flow to the earthing pin E side.
The shield cover 15 includes a ceiling part 15a that covers the upper surface of the circuit board 14 and a skirt part 15b that is fitted on the outer peripheral surface of the connector base 11. In the ceiling part 15a, as shown in FIG. 3C, three through holes of a through hole 151 for the earthing pin E, a through hole 152 for the signal pin SH, and a through hole 153 for the signal pin SC are formed.
To electrically connect the earthing pin E and the shield cover 15 to each other, the through hole 151 for the earthing pin E is formed as a hole having a diameter approximately equal to the diameter of the earthing pin E so as to be capable of being in contact with the earthing pin E, and finally, the earthing pin E is soldered to the shield cover 15. The shield layer consisting of a copper foil solid pattern formed on the back surface side of the circuit board 14 is also connected electrically to the earthing pin E via a predetermined wiring path.
In contrast, the through hole 152 for the signal pin SH and the through hole 153 for the signal pin SC are formed so as to have diameters larger than the diameters of the signal pins SH and SC, respectively, so as to be in non-contact with these pins. In order to keep the leak of a high-frequency magnetic field, which is generated from the wiring part and the like of the capacitor element 14a, into the microphone casing to a minimum when a high-frequency current flows in the capacitor element 14a, the through holes 152 and 153 are preferably formed so as to have as small diameters as possible.
According to this configuration, by the shield layer consisting of a copper foil solid pattern, which is formed on the circuit board 14, and the shield cover 15, a double shield is formed. Also, by the capacitor element 14a mounted on the circuit board 14, the high-frequency current caused by electromagnetic waves can be prevented from intruding through the signal pins SH and SC.
However, between the signal pin SH, SC and the through hole 152, 153, a gap for keeping the signal pin SH, SC in non-contact is present though being small. Also, by the flow of high-frequency current in the capacitor element 14a mounted on the circuit board 14, a high-frequency magnetic field is generated.
This high-frequency magnetic field can be prevented to some degree by an electrostatic shield using the shield cover 15. However, by a leaking high-frequency magnetic field, a high-frequency current is caused to flow by induction in the pattern of printed wiring board for outputting sound signals and the circuit parts mounted thereon in the microphone casing, and thereby noise is sometimes generated.
In the field of condenser microphone, it has been further demanded to reliably prevent the generation of noise caused by extraneous electromagnetic waves. Accordingly, an object of the present invention is to provide an output connector for a condenser microphone which has a function of shielding electromagnetic waves more reliably.